The contact-mediated interactions between cells play an important role in generating and maintaining the multicellularity characteristic of higher animals and plants. When these interactions are disrupted serious aberrations of growth or development may result. Certain microbial systems offer the opportunity to study these interactions in the context of an experimental system that is available to biochemical and genetic analysis. The myxobacteria undergo the most complex of known prokaryote life cycles, throughout which they manifest a variety of cell-cell interactions. They have been thoroughly domesticated so they can be subjected to the sorts of genetic, molecular and biochemical analysis that are characteristic of work with bacteria. In particular, our efforts focus on characterizing various cell surface antigens and attempting to understand their role in cell-cell contact interactions. We have generated a large number of monoclonal antibodies against these antigens and have used them to define which antigens are involved in the interactions, to isolate the antigens, and to isolate mutants deficient in the antigens. We have used cholera toxin to show that a sensory transduction system characteristic of eukaryotic organisms seems to be present in Myxococcus xanthus. We are attempting to determine the role played by this system in transducing the contact signals perceived by the cell surface.